The invention relates to a rotary drill bit for drilling oil and gas wells and, more particularly, to a drill bit suitable for up to hard formations having cutting preforms of polycrystalline diamonds on tungsten carbide substrates.
In rotary drilling, the bit is fixed on the end of a rotating drill pipe inside a casing, the drill pipe being lowered as the drilling progresses. A heavy artificial substance known as drilling mud is circulated down through the drill pipe, out through the bit and back up the casing to remove rock fragments. The drilling mud cools the bit, washes the cutting elements so they present a clean cutting face where the cutting takes place and, as indicated, lifts or carries debris resulting from the drilling to the surface. For the drilling mud to carry out these functions, it is necessary that its velocity through the bit's fluid entrances and channels be high without causing an undesirably high back pressure so it moves quickly across the face and is discharged rapidly and efficiently up the junk slots. It is important clogging of the bit be prevented with rapid removal of cuttings and also that undue stress be avoided.
In lowering bits into a well where cutter elements extend normally from the bit's sides, damage to the casing or the bit or both may occur.
Drill bits, particularly larger drill bits for higher gages, are heavy and difficult to manufacture, requiring comparative large equipment for machining, welding and heat-treating. This is especially so for the head of the bit because of the need for applying gage pads to stabilize and properly position same.
Another problem has been the retention of the cutter preform elements which must be secured to the crown of the bit in an extremely rigid manner. The breakage of any such elements increases the cutting load of the next following cutting element in the set which is then more likely to break or be subjected to greater wear and abuse. These problems tend to reduce the average lifetime of the drill bit which is reflected in decreased efficiency and higher costs for the drilling operations.
Rapid penetration of the earth and longer runs improve substantially the efficiency of the drilling operations. Preform cutting elements as disclosed in U.S. Pat. Nos. 3,745,623, 3,767,371, 4,109,737 and 4,156,329 have been utilized by the inventor and others to provide improved penetration in soft to medium-hard formations such as salt, shale, anhydrite, carbonates, marls, clay and sandstones. But when drilling in harder formations, difficulties have been encountered. Solutions to such problems have been suggested as disclosed, for example, in U.S. Pat. No. 4,006,788 to L. Garner, which applies to a rotary drill bit provided with rolling cone cutters. This patent spells out the need for rotary rock bits which work well in various types of formations, soft or hard, encountered in deep wells where it is highly desirable to penetrate long distances before changing bits.